The present invention relates to a method for regenerating vanadium-phosphorus-oxygen (VPO) catalysts used in the partial oxidation of hydrocarbons to prepare dicarboxylic acids and anhydrides.
It is known that VPO catalysts produce high yields of dicarboxylic anhydrides by partial oxidation of hydrocarbons. It has been found that the yield of product diminishes with time due to a reduction in the selectivity of the catalyst. Various theories of the reason and methods for overcoming the loss in selectivity of catalyst in situ have been devised.
U.S. Pat. Nos. 3,296,282 and 3,474,041, issued to Ralph O. Kerr, disclose that the VPO catalyst may be regenerated and stabilized by adding specified organophosphorus compounds to the catalyst. The organophosphorus compound may be added with or without the hydrocarbon, either intermittently or continuously. It was found that during the use of the catalysts a portion of the phosphorus is removed from the catalyst, thereby disrupting the initial ratio of P:V in the catalyst, and the added phosphorus compound replaced a portion of the lost phosphorus to thereby maintain the catalyst close to its original selectivity and extending the useful life of the catalyst. A similar approach was disclosed in British Patent Specification 1,291,354 and U.S. Pat. Nos. 3,906,008 and 3,975,407.
Another approach to the same problem, i.e., the disruption of the ratio of P to V by phosphorus loss in the catalyst, is the removal of some of the vanadium. A process using this method is disclosed in U.S. Pat. Nos. 4,020,174; 4,090,807 and 4,094,816 issued to Partenheimer. The use of various organic and inorganic halogen compounds are disclosed to regenerate the catalyst. After treatment with alkyl halides, the patents suggest a steam treatment to cause a water gas reaction to remove residual carbon deposited by the alkyl halide.
U.S. Pat. No. 4,111,832 discloses the use of aqueous ammonia and/or amine to at least partially dissolve the catalyst and thereafter redepositing the catalytic components from the solution.
British Patent Specification 1,464,198 discloses the regeneration of VPO oxidation catalysts by the addition of alkyl esters of orthophosphoric acid and European Patent 123,467 using the same type compounds to reactivate the VPO catalysts.
British Patent Specification 1,439,489 discloses the regeneration of VPO catalyst with a reducing agent such as hydrogen, carbon monoxide, methane or hydrogen sulfide.
U.S. Pat. Nos. 4,515,899 and 4,596,878 disclose regeneration of VPO catalysts and life extension by phosphorus compound treatment followed by steam treatment. The steam was believed to redistribute the deposited phosphorus compounds throughout the bed.
Each of these prior art methods may either require multiple steps such as phosphorus/steam treatment, adding or removing catalyst components or otherwise carrying out relatively complicated in situ operation.
The terms "regenerate" and "reactivate" have been used in the prior art to describe various results from the various treatments. The term "regeneration" is used herein to mean that a higher yield of product is obtained after treatment. The treatment may reduce the activity (the term "activity" as used herein, means the ability of the catalyst to convert hydrocarbon at a given temperature) of the catalyst while improving the selectivity. The overall result is that a higher yield (conversions.times.selectivity) is obtained, surprisingly without a commensurate increase in bed temperatures usually associated with the prior art methods.
An advantage of the present invention is that substantial improvement in the yield can be obtained without any added activator such as phosphorus compounds. Also this improved yield is obtained without an increase in the temperature of the reaction after treatment. It is an advantage of the present invention that regeneration of the VPO catalyst is obtained without the introduction of any extraneous material into the reaction system. It is a further advantage that the regeneration is obtained using available cheap material.